Generally, a resistor of an IC termination is needed to make signal transmission between devices smooth. When resistances are not matched properly, the transmitted signal may be reflected to cause errors in signal transmission. However, if a fixed resistor is given externally for this matching, resistances cannot be matched properly due to decline of the IC, temperature change, or difference of manufacturing process. For this, a new technique is introduced for controlling the resistance of the IC termination by controlling the number of turned-on transistors among transistors, which are coupled in parallel, to have same resistance as an external reference resistor.
U.S. Pat. No. 6,087,847 issued on Jul. 11, 2000 discloses an IC including a digital feedback control circuit for controlling an impedance of an interface circuit output buffer based on impedance control of non-data signal output buffer that is partially coupled to an external impedance, to suggest a circuit for varying a resistance at an output buffer such as a micro-processor. However, a non-data signal output buffer as shown in FIG. 2 in U.S. Pat. No. 6,087,847 uses an output buffer having same type as an interface circuit output buffer, which has so many logic devices used for impedance control. Therefore, the area for the circuit for controlling the impedance within the IC is to be increased. Further, because U.S. Pat. No. 6,087,847 connects only one of a power voltage and a ground voltage for the output buffer, it cannot be used for resistance control of the termination that is operated by simultaneously turning on switches that is connected to both of the power voltage and the ground voltage.